The thermal performance of a power module is directly linked to the thermal resistance of its material stack.
In typical power modules, the power devices, such as MOSFETs and IGBTs, are soldered to conductive plates. The conductive plates may be electrically isolated from one another because the power devices may be at different potentials. For example, the power module may include a half-bridge circuit that is comprised of two series connected MOSFETs, each of which having a drain contact connected to a respective conductive plate. In such a case, electrical isolation is required because the drains of the MOSFETs have different potentials and may need to be isolated from each other. The isolation may be achieved by forming conductive regions for receiving the power devices on a major surface of an insulated metal substrate (IMS), direct-copper (DBC), or the like or using materials such as SilPads between the power devices and the heatsink to achieve electrical isolation. Such elements contribute to the thermal resistance of the power module. Taking the IMS as an example, an IMS includes an insulating body, such as a ceramic body, having a conductive layer, such as a layer of aluminum, formed on opposing major surfaces thereof. One of the conductive layers may be modified by, for example, etching to include conductive pads for receiving the power devices. Power devices generate heat during operation which can be dissipated from the conductive layer on the opposing surface, especially if a heatsink is attached thereto.
There are disadvantages associated with using an IMS or the like in a power module. For example, an IMS exhibits poor thermal conductivity compared to the rest of the elements in the thermal stack. The poor conductivity is largely caused by the insulating body. Indeed most of the temperature drop appears across the insulating body which, depending on the material, can constitute 50% to 80% of the RthJS of the module.
Also, materials such as IMSs are expensive thus raising the cost of the power module.